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Wireless How To

Social Engineering

As much as you may hate to admit it, you may not be able to solve your wireless problems by yourself. Since the heart of the problem is caused by lack of coordination (and communication) among users trying to run a number of wireless LANs in too small an area, the most effective solution would be to apply the design techniques used in large multi-AP WLANs.

You may be surprised at the willingness of people to work together to solve a common problem—especially if they don't really have to do much. Put up a sign and call a meeting of your apartment building, dorm, or neighborhood. If you've got the a WLAN problem, chances are others do too.

Once you've got the interested parties together, the main order of business is to see if you can work out a satisfactory channel assignment scheme. If there are only three APs involved, the job is pretty simple. But if you're dealing with more APs, you'll have to put in a more effort.

Make a diagram of the APs as close to scale as you can get it. Once you have the APs located, it's just a matter of juggling channel assignments so that APs using the same channels have the lowest signal strength with respect to each other. Since signal strength is primarily related to distance, a practical approximation of this rule is to locate same-channel APs as far apart as possible.

In some cases, building construction and other RF-unfriendly obstacles like trees, water, screens, etc. may allow you to bend this "farthest-distance" rule a bit. In multi-story situations, be sure to think in three dimensions because radio waves travel in all directions!

Once you have your channels assigned, but sure to assign unique SSIDs to each AP. Though you want to treat the APs as one big WLAN from a channel assignment point of view, you still want multiple, separate WLANs from an operational point of view. Unique SSIDs will keep clients from trying to roam where they're unwanted.

Finally, if you find that your neighbors are unfamiliar with the wonders of WEP / WPA, MAC address association control (filtering) and other WLAN security features, help them get that set up too. With the problem of wireless-security throughput essentially gone from current-generation WLAN equipment, there's no reason to run wide-open WLANs if you don't want to.

Use WPA2 / AES security with a strong password if you can, i.e. all your devices support it. You especially want to use WPA2 / AES if you're running 802.11n gear, since you'll get only 802.11g speeds (54 Mbps maximum link rate, around 20 Mbps best-case real throughput) if you use WEP or WPA / TKIP.

What Doesn't Help

When faced with a wireless LAN that won't behave, people will try most anything in search of a cure. But some "solutions" don't really help and can actually hurt your (and your neighbors') WLANs performance. Let's look at some "solutions" commonly suggested.

Turning on WEP, WPA, WPA2 / Using authentication
Encryption methods such as WEP and WPA or using one of the many 802.1x authentication methods will keep unwanted clients from associating with your wireless LAN, but not from trying! Preventing association keeps clients from using significant amounts of your WLAN's bandwidth, but association attempts - especially in areas with many wireless clients - can still cause performance to degrade.

Put another way, use of encryption doesn't do anything to the radio signal itself, but only to the information it is carrying. So while I recommend enabling WPA2 for security reasons, it doesn't do anything to reduce the effect of too many radios in too small a space.

Turning off SSID broadcast
It's not the security precaution that some articles make it out to be, and it won't really help keep your clients from trying to associate with other wireless LANs. But turning off the broadcast of your AP's SSID could help your neighbors' clients to stay on their own WLAN. But again, change your default SSID, too, because if your neighbor's laptop detected and saved it in its "Preferred network" list at some point, it will still look for it during its association attempts.

Selecting 11g-only or 11n-only mode; Disabling "protection"
Owners of 802.11g and N APs may have a few other knobs to fiddle with, depending on the controls they expose. Some APs allow disabling of the 802.11b "Protection" mechanism that enables slower 11b clients to interoperate with faster 11g APs. And 11n routers have similar controls that control backward-compatibility features for 11g.

The only thing shutting off these protection mechanisms will do, however, is negatively affect the performance of your WLAN. If your AP can't "hear" your slower clients that are trying to communicate with it, they won't be able to connect. And the clients will still be generating traffic as they constantly try to seek out an AP to associate with.

Boosting your signal
As pointed out in How To Fix Your Wireless Network - Part 3, signal boosting at best solves half the problem, since it can only help a client to "hear" an AP better and not vice-versa. Though I have to admit that boosting might help keep your client from straying by providing a stronger signal to latch onto, I would use this only as a last resort. Solving a problem by passing it on to someone else (your neighbor) isn't really a solution.

Using Channel Bonding
Atheros' Super-G was the first bandwidth-hogging "channel bonding" technique, and 802.11n has 40 MHz mode baked into the standard. Neither should be used in crowded 2.4 GHz band WiFi environments because there simply isn't enough available bandwidth.

While you may think that you are drowning out neighboring WLANs, you're just raising the RF noise level, causing problems for everyone. Leave your N router's defaults alone and use only 20 MHz bandwidth mode in 2.4 GHz.

Using Channels other than 1, 6 and 11 in 2.4 GHz
Let's bring back an earlier picture to show why this doesn't help.

2.4 GHz band adjacent channel overlap - again

Figure 17: 2.4 GHz band adjacent channel overlap - again

If you're using Channel 1 and your neighbor is using Channel 2, you're both putting plenty of power into each other's channel. Even the best receivers have difficulty dealing with this level of "adjacent channel interference". Whatever a receiver can't understand, ends up as "noise", which can reduce performance.

On the other hand, when both your WLANs use the same channel, the CSMA/CA mechanism described earlier, as well as other Wi-Fi coordination techniques, can operate as intended. Although you both will be contending for a share of the same spectrum (and bandwidth), your requests can be coordinated for most efficient sharing. In other words, you may not get the speed you want, but you'll get reliable operation.

Closing Thoughts

The 802.11 protocol at the heart of Wi-Fi is amazingly robust and capable of supporting dozens, if not hundreds of stations in a given area. The key to successful operation, however, is cooperation, not competition.

The old saw of "thinking globally, and acting locally" really is the best approach to having multiple wireless LANs operate successfully. Now that you know how, get out there and lead your wireless neighborhood to wireless networking harmony!

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